The goal of this proposal is to study the structure/function relationship of telomerase. This enzyme protects the ends of chromosomes, maintains their length during cellular division, and is selectively activated in most humans cancers. Through these activities, it maintains genomic integrity, promotes cellular immortalization, and sustains the proliferating status of transformed cells: its pharmacological inhibition may very well provide a new avenue to cancer treatment. Telomerase is a ribonucleoprotein composed of a non-coding RNA, a protein enzymatic component and additional essential protein subunits. Very little is known structurally on telomerase or its components, yet this knowledge, and an understanding of the molecular interactions between its protein and RNA components, are essential to understand its function and mechanism. We propose to determine the structure of telomerase-associated proteins and of key functional domains of its RNA component. The structural information will be integrated into a biochemical and functional context, through collaborations with biochemists and molecular biologists (Gottchling, Lingner and Rhodes), who will provide the breadth of expertise required to answer, at the molecular level, fundamental questions on how this enzyme functions. The enzymatic activity of telomerase (TERT) is homologous to viral reverse transcriptases, while the RNA (TER) provides the template and a landing dock for RNP assembly. Different biochemical functions map to specific domains of TER and to the proteins that associate with them. Because the telomerase holoenzyme is large and complex, I propose to build up its structure from its constituent parts, based on the "divide-and-conquer" principle. Specifically, I propose to study protein-RNA complexes responsible for: 1. RNA processing, stability and assembly of the mature RNP; 2. Targeting of the mature telomerase to the telomeres; 3. Binding and activation of TERT. The research plan will be based on: 1. The analysis by NMR of the structure and dynamics of the domains of TER responsible for each of these functions; 2. The determination of the structure of the proteins that interact with such domains; 3. The biochemical and structural characterization of the resulting RNA-protein complexes; 4. The design and execution of new biochemical and functional experiments, grounded onto the structural results, that will address the biological function of these protein-RNA complexes.